Method For Removing A Coating From A Component

ABSTRACT

Method for removing coatings which have been applied to components such as turbine blades. After the end of the product life cycle of the turbine blades has been reached, these coatings can be removed to reuse the turbine blades after recoating. The coating, containing in particular chromium oxide compounds is removed via a stripping bath by adding alkanolamine compounds or salts containing such compounds as the inhibitor. These compounds advantageously prevent new chromium oxide compounds from being produced during the stripping of the turbine blades or the chromium oxide compounds present in the coating are removed effectively, so that the chromium oxide compounds cannot have an adverse influence on the removal rate of the stripping process. Advantageously reduced treatment times can thereby be achieved for the stripping process. By adding 2% triethanolamine, the treatment time in hydrochloric acid for example can be reduced to below one hour.

The invention relates to a method for removing a coating containing chromium and/or chromium oxide compounds from a component with the aid of a stripping bath.

According to EP 1 094 134 A1, such a stripping operation can be carried out for example by means of an electrochemical process. This involves immersing the component from which the coating is to be stripped in an electrolyte and thereby forming the working electrode. By applying a voltage to the corresponding counter electrodes, the coating on the component that is to be stripped is dissolved.

Another method for stripping coatings from components is described in EP 1 314 797 A2. This is a chemical stripping method. The component from which the coating is to be stripped is immersed in a solution comprising hydrochloric acid and an inhibitor reducing the extent to which the base material of the component is attacked. The stripping bath formed by the hydrochloric acid makes the coating dissolve.

With the methods mentioned it is possible for example to strip coatings from turbine blades when they reach the end of their intended service life. This is because subsequent recoating allows the turbine blades to have a further life cycle, for which reason there is great interest in the effective stripping of coatings from turbine blades. When stripping coatings from turbine blades, it is necessary to remove not only the coatings that have been used, such as MCrAlY coatings or thermal protective coatings (known as thermal barrier coatings, hereafter TBCs), but also the contaminants that have occurred on the coating over the time during which the turbine blade has been in operation, preferentially comprising complex crystalline compounds. Compounds of the Ca—Mg—Al—Si—O system (hereafter referred to as CMASs) may be mentioned in this connection. Other compounds that are difficult to remove comprise what are known as thermally grown oxides (hereafter TGOs) on the base material of the coated component, such as for example Cr₂O₃ or Cr_(x)Co_(y)O compounds. These compounds have a high chemical stability, for which reason they can only be removed by the stripping bath if they are left for a considerable time. Furthermore, such compounds may also be formed during the stripping process, causing the stripping procedure to take even longer. The compounds increasingly cover the surface of the component to be stripped, for which reason the removal rate during stripping drops as the treatment time increases. According to EP 1 314 797 A2, it is therefore proposed to carry out the stripping process in a number of steps, with the surfaces being mechanically worked between each of the treatment steps, so that the contaminants that occur or have not yet been removed can be removed.

The object of the invention is to provide a method for removing coatings containing chromium and/or chromium oxide compounds which allows short treatment times for the stripping process.

This object is achieved according to the invention by a method for removing a coating containing chromium and/or chromium oxide compounds from a component in which the coated component is introduced into a stripping bath and is removed again after the treatment time has elapsed, an alkanolamine compound or a salt containing such a compound being added to the stripping bath as an inhibitor. This is because it has surprisingly been found for alkanolamine compounds and salts containing them as inhibitors that they not only reduce an attack on the base material of the coated component by the stripping bath but also suppress the formation of TGOs or the dissolving of TGOs and CMASs. This advantageously allows a stripping process with comparatively high removal rates to be maintained over a longer treatment time than is possible with the inhibitors that are used according to the prior art. This allows the method according to the invention to be used more cost-effectively, in particular there is no need for the stripping procedure to be interrupted for an intermediate mechanical working step, or at least allows the treatment times between these mechanical working steps to be prolonged. This advantageously reduces the overall time that is necessary for performing the stripping process, for which reason the method can be used more cost-effectively. Furthermore, instances of the base material of the coated component being damaged as a result of mechanical working steps can be avoided, whereby the number of reject components during stripping is reduced, which additionally contributes to improved cost-effectiveness of the method. It has been found to be advantageously particularly effective to use triethanolamine or one of its salts, such as triethanolamine borate, as the inhibitor.

It is advantageous to add carbonic acids and/or aldehyde compounds and/or unsaturated alcohols as further inhibitors, the effect of which as an inhibitor during stripping is already known. The mixtures of inhibitors advantageously improve the overall effect of the stripping bath as a whole.

The method according to the invention can be used particularly advantageously for the stripping of coatings from turbine blades, it being possible in particular for MCrAlY coatings and/or thermal barrier coatings to be removed from the turbine blades. The MCrAlY coating often serves here as an intermediate layer between the turbine blade and the thermal barrier coating (TBC). It goes without saying that said coating systems can also be removed from any other components if necessary (for example from feed pipes of gas turbines).

Further details of the invention are described below with reference to the drawing. The single FIGURE shows a diagram which represents the course of various exemplary embodiments of the method according to the invention. The exemplary embodiment of the method was that of removing an MCrAlY coating from turbine blades that have been in operation, a five percent hydrochloric acid being used as the stripping bath. Furthermore, in a method conducted in parallel, triethanolamine of a one percent or two percent concentration was added. During the treatment time, the stripping bath was exposed to ultrasound at a frequency of 36 kHz.

The exemplary embodiments of the method according to the invention involved electrochemical stripping, the potential at the turbine blade being kept at 0.100 V SCE and 0.340 V SHE. The diagram shows the current I in amperes A measured at the turbine blade over the treatment time t in hours h.

This current can be used as a direct measure of the removal rate at the turbine blade.

It is found that, with the addition of one percent or two percent triethanolamine (TEA), the current density for dissolving can be increased by a multiple in comparison with the use of pure hydrochloric acid. This allows removal of the coating from the turbine blade to be achieved in a correspondingly shorter time. This is evident in the drawing from the drop in current. The necessary treatment times can be reduced as a result to below one hour. 

1-5. (canceled)
 6. A method for electrochemically removing a coating containing chromium and/or chromium oxide compounds from a component in which the coated component is introduced into a stripping bath and is removed again after the treatment time has elapsed, an alkanolamine compound or a salt containing such a compound being added to the stripping bath as an inhibitor, characterized in that triethanolamine or one of its salts is added as the inhibitor, the coating being removed from a turbine blade.
 7. The method as claimed in claim 6, characterized in that carbonic acids and/or aldehyde compounds and/or unsaturated alcohols are added as further inhibitors.
 8. The method as claimed in claim 6, characterized in that the coating is an MCrAlY coating and/or a thermal barrier coating.
 9. The method as claimed in claim 7, characterized in that the coating is an MCrAlY coating and/or a thermal barrier coating. 